Postmenopausal women are at higher risk of coronary heart disease (CHD) than premenopausal women, even after controlling for age-effects. However, estrogen replacement therapy significantly lowers the risk of CHD in postmenopausal women. With the study, we propose to determine the kinetics of apolipoprotein A-1, the major protein component of high density lipoproteins, in postmenopausal women undergoing a placebo-controlled, randomized treatment with each of the following regimens: a. placebo; b. 0.625mg/d conjugated estrogens; c. 0.625mg/d conjugated estrogens + 2.5 mg/d medroxyprogesterone acetate. The kineetics of apo A-1 will be determined by stable isotope techniques. The low dose estrogen (b) and the estrogen-progestin combination (c) are frequently prescribed to postmenopausal women as hormone replacement therapy, but there are metabolism and apolipoprotein kinetics.